Research

Wurtzite

The Advanced Inorganic Materials research group (formerly the semiconductors and ceramics group) carries out a variety of work on thin film semiconductors, with particular focus on semiconductors used for photovoltaics (PV). In particular a large part of the work in the group is on II-VI materials such as CdTe for solar cells.

We are also carrying out research into the preparation and measurement of various electroceramic materials for humidity sensors and thermistors (both negative and positive temperature coefficient materials).

New research in the group is based around the investigation of Gallium Nitride and Indium Gallium Nitride quantum well structures using simultaneous cathodoluminescence and scanning transmission electron microscopy. For further information please contact Dr Douglas Halliday.

1. Polycrystalline solar cells

This research group is well known for work on the characterisation of efficiency limiting effects in thin film solar cells. You can find out more by looking at our publications page, searching the published litertature or by contacting Dr Douglas Halliday, the solar cell materials group leader.

The CdS/CdTe solar cell

Advantages of CdS/CdTe

Currently, the semiconductor most widely used in solar cells is single-crystal silicon. Because of the cost involved in producing the bulk material, cells produced by this method are prohibitively expensive for all but the smallest scale or most specialised applications (such as on calculators and satellites). Higher efficiencies have been produced by using single-crystal III-V semiconductors and more elaborate constructions (e.g. multi-quantum wells), but this advantage has always been more than offset by the resultant increase in cost.

The thin-film cadmium telluride / cadmium sulphide solar cell has for several years been considered to be a promising alternative to the more widely used silicon devices. It has several features which make it especially attractive:

The cell is produced from polycrystalline materials and glass, which is a potentially much cheaper construction than bulk silicon.

The chemical and physical properties of the semiconductors are such that the polysilicon thin-films can be deposited using a variety of different techniques (see below).

CdTe has a bandgap which is very close to the theoretically-calculated optimum value for solar cells under unconcentrated AM1.5 sunlight.

CdTe has a high absorption coefficient, so that approximately 99% of the incident light is absorbed by a layer thickness of only 1µm (compared with around 10µm for Si), cutting down the quantity of semiconductor required.

A concern often expressed about CdS/CdTe solar cells is the effect on health and the environment of the cadmium used. However, the thinness of the films means that the amount of active material used is relatively small; it has been estimated that even if CdTe solar cells were to provide more than 10% of the world's energy requirements, this would still only account for less than a tenth of the world's cadmium usage . To put the risk into perspective, B.P. Solar modules have been reported to have passed the appropriate U.S. Environmental Protection Agency tests, whereas fluorescent tubes (containing mercury) and computer screens (containing lead) do not.

Cell construction

The CdTe/CdS solar cell is based around the heterojunction formed between n-type CdS and p-type CdTe. The basic composition of the cell can be seen in Figure 4.

Figure 4: CdS/CdTe solar cell (not to scale)

The functions of the different layers are as follows:

Glass The solar cell is produced on a substrate of ordinary window glass, because it is transparent, strong and cheap. Typically around 2-4 mm thick, this protects the active layers from the environment, and provides all the device's mechanical strength. The outer face of the pane often has an anti-reflective coating to enhance its optical properties.

Transparent conducting oxide Usually of tin oxide or indium tin oxide (ITO), this acts as the front contact to the device. It is needed to reduce the series resistance of the device, which would otherwise arise from the thinness of the CdS layer.

Cadmium sulphide The polycrystalline CdS layer is n-type doped (as CdS invariably is), and therefore provides one half of the p-n junction. Being a wide band gap material (Eg ~ 2.4 eV at 300K) it is transparent down to wavelengths of around 515 nm, and so is referred toas the window layer. Below that wavelength, some of the light will still pass through to the CdTe, due the thinness of the CdS layer (~ 100 nm).

Cadmium telluride The CdTe layer is, like the CdS, polycrystalline, but is p-type doped. Its energy gap (1.5 eV) is ideally suited to the solar spectrum, and it has a high absorption coefficient for energies above this value. It acts as an efficient absorber and is used as the p side of the junction. Because it is less highly doped than the CdS, the depletion region is mostly within the CdTe layer. This is therefore the active region of the solar cell, where most of both the carrier generation and collection occur. The thickness of this layer is typically around 10 µm.

Back contact Usually of gold or aluminium, the back contact proves a low resistance electrical connection to the CdTe. P-type CdTe is a notoriously difficult material on which to produce an ohmic contact, and so the junction will inevitably display some Schottky diode (rectifying) characteristics. Due to its high conductivity, the metal layer needs only be a few tens of nanometres in thickness.

Since the active layers of the device are those on top of the glass substrate, this construction is referred to as a superstrate configuration.

Deposition techniques

The polycrystalline layers of CdS and CdTe can deposited by a number of different methods, including, amongst others, those outlined below:

Physical vapour deposition (PVD) (or evaporation) involves the vaporisation in a vacuum of a source of either the compound (CdS or CdTe) or the separate elements (Cd + S or Cd + Te). The resulting vapours recombine on the surface of the substrate (which can be heated, but is still much cooler than the source) to deposit the required polycrystalline material. The stoichiometry of the deposited layer is difficult to control accurately, as it depends strongly on the equilibrium vapour pressures of the elements, as well as the stoichiometry of the source material.

Close-space sublimation (CSS), which has been used to produce the highest efficiency cells so far , is based on the reversible dissociation of the materials at high temperatures e.g.

2CdTe(s) = Cd(g) + Te2(g)

The source is of a large area and is positioned close to the substrate. The substrate is maintained at a high temperature (but below that of the source) such that the elemental vapours will not become deposited on the substrate but the compound form will, due to its lower equilibrium vapour pressure.

Chemical vapour deposition (CVD) can also be used to deposit both semiconductors. It involves chemical reactions between vapours to produce the required species which then condense on the substrate to form the compound. One variation of this method, Metal-Organic CVD (MOCVD), uses metallo-organic precursors: this is an especially widespread technique, as it produces thin films with very good optical and electronic properties.

Chemical bath deposition is sometimes used for depositing CdS films, and involves producing the required ions in a solution by chemical means, which combine and precipitate out onto the substrate if the required equilibrium conditions are met. For example, cadmium ions can be produced by the hydrolysis of Cd(OH)2:

Cd(OH)2 = Cd2+ + 2(OH)-

and sulphide ions from an alkaline aqueous solution of thiourea:

(NH2)2CS + OH- = CH2N2 + H2O + HS-

HS- + OH- = H2O + S2-

However, this method can not used for CdTe, due to the difficulty of synthesising tellurides

Electrodeposition may also be used to deposit many semiconductor materials at low temperature from solution.

3. Electroceramics

Ceramic materials are polycrystalline, nonmetallic inorganic substances prepared by solid state chemical reaction followed by a sintering process at elevated temperatures.

Electroceramics are ceramic materials with a certain fixed electrical property, e.g. insulators, ferroelectric materials, highly conductive ceramics, electrodes, etc. According to the chemical composition of the monophase compound electroceramic can be oxide or nonoxide (nitride, boride, carbide, etc.). Oxide electroceramics are the ones most often used for producing sensors, electrodes and electronic devices.

According to their composition, oxide electroceramics can be subdivided into:

single oxides

two or more oxides (often refered to as binary, ternary, .... systems)

The various single and complex oxides can combine to form solid solutions which are of great interest.

Electroceramic research at Durham is mainly focused on electrical properties of ternary and higher complex systems and could be divided broadly into the following areas: